In communication networks, for example optical communication networks, single-fiber working consists of using a single fiber for bidirectional transmission, instead of using a pair of fibers, one for each direction. One factor for adopting the single-fiber technique is the cost of fiber deployment or renting. The actual cost of a cable and of trenching or aerial mounting a cable is relatively insensitive to the number of fibers in the cable. However, the cost of splicing and the cost of components such as splitters, connectors, couplers and splice enclosures are directly related to the fiber count.
Single-fiber solutions can therefore help reduce the capital and labor costs associated with lighting up a fiber while effectively doubling the number of available fibers. This may be exploited in access networks or in mobile front haul arrangements to reach nodes such as remote radio units (RRU).
Besides exploiting single-fiber transmission, access networks are also mostly based on passive solutions (for example providing no amplification), and as such the insertion loss of passive components (such as multiplexers, add/drop filters, etc.) should be minimized in order to improve the system reach and capacity.
Typically, two functions are performed at Optical Add Drop Multiplexer (OADM) nodes. One function is to ADD/DROP desired channels (typically in a given band) in each direction of a ring (e.g. termed an East direction and a West direction) or a linear connection. Another function is to ADD/DROP the optical supervisory channel (OSC), which is typically in a different band than the one used by service channels. A band may be considered as a frequency band, e.g. a range or set of contiguous or non-contiguous frequencies.
FIG. 1 shows an example of a fixed Optical Add Drop (OAD) filter, for example in the form of a Thin Film Filter (TFF). The filter of FIG. 1 comprises a 3-port device 100, comprising a common port 101, and ADD/DROP port 103 and an express port 105.
The common-to-express path (i.e. between the common port 101 and the express port 105) is based for example on a filter function (illustrated schematically by the dotted line 104) providing reflection of optical signals on a pass-through path. The filter function 104 in reflection provides for low isolation from optical signals on other paths (i.e. to/from the ADD/DROP port). The common-to-drop path (i.e. between the common port 101 and the ADD/DROP port 103) and the add-to-common path (i.e. between the ADD/DROP port 103 and common port 101) are based on the filter function 104 providing transmission of optical signals to/from the ADD/DROP port.
The filter function 104 may provide for a high isolation of the transmitted optical signals. The filter function 104 allows certain wavelengths or a band or set of wavelengths to pass through the filter, while the filter function 104 reflects certain wavelengths or a band or set of wavelengths between one port and another. It is noted that a pass filter may comprise one or more individual filters, for example one filter for adding a particular wavelength and another filter for dropping a particular wavelength.
In such a filter as shown in FIG. 1, to keep the insertion loss on a pass-through path (i.e. between the common port 101 and the express port 105) as low as possible, one such filter can be used to add/drop a composite band (with all the local channels) to the fiber, with additional single-channel filters being appended on the add/drop path (i.e. appended to the add/drop port 103) to select the individual wavelengths.
FIG. 2 shows an example of how a network node, functioning as a OADM node 200, may be implemented using two 3-port OAD filters 100 for the ADD/DROP channels (one for the East and one for the West direction), indicated by WDM band. In addition, the node 200 further comprises two 3-port OAD filters 100 for the OSC channel (again for East and West directions), marked as 1490 nm/1510 nm. The OAD filters 100 have ports 101,103,105 corresponding to the ports described in FIG. 1. The OADM node 200 is connected to one or more further nodes by an optical connection 115, e.g. optical fiber.
The OAD filter 1001 is provided for adding/dropping channels (e.g. OSC channels) in a West direction, while the OAD filter 1004 is provided for adding/dropping channels (e.g. OSC channels) in an East direction. The OAD filter 1002 is provided for adding/dropping data (i.e. service) channels in a West direction, while the OAD filter 1003 is provided for adding/dropping data (i.e. service) channels in an East direction.
An add/drop filter 111, for example comprising N single-channel filters, is provided for adding and dropping the data channels. It is noted that in this example the add/drop filter 111 is split towards the OAD filters 1002 and 1003, which represents the case of ring protection. It is noted that two independent filters 111 could also be provided for the OAD filters 1002 and 1003, for example if independent traffic is required for West and East directions. Filters 1091 and 1092 are appended to the respective add/drop ports 1031, 1034 of the respective OAD filters 1001, 1004, i.e. in order to help reduce insertion loss on the fiber 115 itself. In other words, the filters 1091 and 1092 are not coupled directly to the fiber 115, thus reducing the insertion loss on the fiber 115 itself due to these filters.
In FIG. 2 it can be seen that a filter 1091, for example a coarse wavelength division multiplexing (CWDM) filter, provides for adding a first wavelength, e.g. having a value 1490 nm, to the West direction, and provides for dropping a second wavelength, e.g. having the value 1510 nm from the West direction. In contrast, a filter 1092, for example a CWDM filter, adds the second wavelength e.g. having the value 1510 nm to the West direction, and drops the first wavelength having the value 1490 nm from the West direction.
A disadvantage with the arrangement of FIG. 2 is that there are four cascaded filters per node 200, i.e. 1001 to 1004, thus increasing the insertion loss of the node. The resulting insertion loss of a chain of such OAD nodes 200 (for example 4 or 8 nodes in a typical network) can strongly affect the available reach of the network. For example, considering 8 nodes and an insertion loss of 0.5 dB for each express path, such an arrangement would result in an insertion loss of 8×4×0.5=16 dB, which is a significant insertion loss.